Generally, high-purity polycrystalline silicon is used as a basic material for manufacturing semiconductor devices or solar cells. The polycrystalline silicon is prepared by thermal decomposition and/or hydrogen reduction of highly-purified silicon atom-containing reaction gas, thus causing the continuous silicon deposition on silicon particles.
For mass production of polycrystalline silicon, a bell-jar type reactor has been mainly used, which provides a rod-type polycrystalline silicon product with a diameter of about 50-300 mm. However, the bell-jar type reactor, which consists fundamentally of the electric resistance heating system, cannot be operated continuously due to inevitable limit in extending the maximum rod diameter achievable. This reactor is also known to have serious problems of low deposition efficiency and high electrical energy consumption because of limited silicon surfaces and high heat loss.
Alternatively, a fluidized bed reactor has recently been developed to prepare granular polycrystalline silicon with a size of 0.5-3 mm. According to this method, a fluidized bed of silicon particles is formed by the upward flow of gas and the size of the silicon particles increases as the silicon atoms deposit on the particles from the silicon atom-containing reaction gas supplied to the heated fluidized bed.
As in the conventional bell-jar type reactor, the fluidized bed reactor also uses a silane compound of Si—H—Cl system such as monosilane (SiH4), dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), silicon tetrachloride (SiCl4) or its mixture as the silicon atom-containing reaction gas, which usually further comprises hydrogen, nitrogen, argon, helium, etc.
For the silicon deposition, the reaction temperature (i.e., temperature of the silicon particles) should be maintained high. The temperature should be about 600-850° C. for monosilane, while being about 900-1,100° C. for trichlorosilane which is most widely used.
The process of silicon deposition, which is caused by thermal decomposition and/or hydrogen reduction of silicon atom-containing reaction gas, includes various elementary reactions, and there are complex routes where silicon atoms grow into granular particles depending on the reaction gas. However, regardless of the kind of the elementary reaction and the reaction gas, the operation of the fluidized bed reactor provides a granular polycrystalline silicon product.
Here, smaller silicon particles, i.e., seed crystals become bigger in size due to continuous silicon deposition or the agglomeration of silicon particles, thereby losing fluidity and ultimately moving downwards. The seed crystals may be prepared or generated in situ in the fluidized bed itself, or supplied into the reactor continuously, periodically or intermittently. Thus prepared bigger particles, i.e., polycrystalline silicon product may be withdrawn from the lower part of the reactor continuously, periodically or intermittently.
Due to the relatively high surface area of the silicon particles, the fluidized bed reactor system provides a higher reaction yield than that by the bell-jar type reactor system. Further, the granular product may be directly used without further processing for the following-up processes such as single crystal growth, crystal block production, surface treatment and modification, preparation of chemical material for reaction or separation, or molding or pulverization of silicon particles. Although these follow-up processes have been operated in a batchwise manner, the manufacture of the granular polycrystalline silicon allows the processes to be performed in a semi-continuous or continuous manner.
The increase in the productivity of the fluidized bed reactor is required for low-cost manufacture of granular polycrystalline silicon. For this purpose, it is most effective to increase the silicon deposition rate with low specific energy consumption, which is obtainable by continuous operation of the fluidized bed reactor under high pressure. For continuous operation of the process with the fluidized bed reactor, it is essential to secure the physical stability of the reactor components.
Unlike conventional fluidized bed reactors, serious limitations are encountered in material selection of the components of the fluidized bed reactor for preparing polycrystalline silicon. Especially, considering the desired high purity of the polycrystalline silicon, the material selection of the fluidized bed wall is important. The reactor wall is weak in physical stability because it is always in contact with silicon particles fluidizing at high temperatures, and is subject to the irregular vibration and severe shear stress caused by the fluidized bed of the particles. However, it is very difficult to select an appropriate material among the high-purity non-metallic inorganic materials that are capable of enduring a relatively high pressure condition, because metallic material is not appropriate because of high reaction temperature and chemical properties of the reaction gas. For this reason, the fluidized bed reactor for manufacture of polycrystalline silicon inevitably has a complicated structure. It is therefore common that a reactor tube made of quartz is positioned in an electrical resistance heater for heating the silicon particles, and both the reactor tube and the heater are surrounded by a metallic shell. It is preferred to fill an insulating material in between the heater and the reactor shell or outside the reactor shell to reduce heat loss.
For example, U.S. Pat. No. 5,165,908 discloses a reactor system where an electric resistance heater encloses a reactor tube made of quartz, both of which are protected by a jacket-shaped stainless-steel shell and an insulating material is installed outside the shell.
U.S. Pat. No. 5,810,934 discloses a fluidized bed reactor for manufacture of polycrystalline silicon, comprising a reactor vessel, i.e., the reactor tube defining a fluidized bed; a shroud, i.e., a protection tube surrounding the reactor tube; a heater installed outside the shroud; and an outer containment surrounding the heater and an insulating material. This patent emphasizes that the protection tube made of quartz be installed in between the reactor tube and the heater to prevent the crack of the reactor tube and the contamination of its inner space.
Meanwhile, the fluidized bed reactor for manufacture of polycrystalline silicon may have a different structure depending on the heating method.
For example, U.S. Pat. No. 4,786,477 discloses a method of heating silicon particles with microwave penetrating through the quartz reactor tube instead of applying a conventional heater outside the tube. However, this patent still has a problem of a complex structure of the reactor and fails to disclose how to increase the reaction pressure inside the quartz reactor tube.
To solve the above problem, U.S. Pat. No. 5,382,412 discloses a simple-structured fluidized bed reactor for manufacture of polycrystalline silicon, wherein a cylindrical reactor tube is hold vertically by a metallic reactor shell. However, this patent still has problems that the inner pressure cannot be increased beyond atmospheric pressure and the microwave supplying means should be combined with the reactor shell, thus failing to suggest how to overcome the mechanical weakness of the reactor tube that is anticipated at high-pressure reaction.
Therefore, in an embodiment of the present invention there is provided a high-pressure fluidized bed reactor for preparing granular polycrystalline silicon, which comprises (a) a reactor tube, (b) a reactor shell encompassing the reactor tube, (c) an inner zone formed within the reactor tube, where a silicon particle bed is formed and silicon deposition occurs, and an outer zone formed in between the reactor shell and the reactor tube, which is maintained under an inert gas atmosphere, and (d) a controlling means to keep the difference between pressures in the inner zone and the outer zone being maintained within the range of 0 to 1 bar, thereby enabling to maintain physical stability of the reactor tube and efficiently prepare granular polycrystalline silicon even at relatively high reaction pressure.
Further, in another embodiment of the present invention, there is provided a fluidized bed reactor that can be conveniently applicable to the manufacture of high-purity silicon particles while minimizing the impurity contamination.